Conventionally, a method for detecting a change in volume of a blood vessel resulting from fluctuations and a method for detecting a change in pressure in the blood vessel are known as methods for measuring a pulse. The change in the volume of the blood vessel resulting from fluctuations is typically detected by using an optical method, while the change in the pressure in the blood vessel is typically detected by using a piezoelectric method. In the optical method, a living body is irradiated with a light in a wavelength range of the hemoglobin contained in the blood, and the strength of the transmitted light or reflected light is converted into an electrical signal to thereby detect the pulse. In the piezoelectric method, a piezoelectric sensor is pressed against an artery, and a voltage obtained by the piezoelectric sensor is detected as the pulse.
However, in both the optical method and the piezoelectric method, since it is required to mount the sensor to the body of a subject, the movement of the body of subject will be limited to some extent. Particularly, in the piezoelectric method, in order to perform precise measurement, it is required to employ a pressing means to correctly press the piezoelectric sensor on directly above the artery, and therefore not only the movement of the body of the subject, but also the environment for performing measurement will be considerably limited.
To solve such a problem, a method has been proposed in which the pulse can be detected in a non-contact manner without directly contacting the body of the human body with the sensor. The non-contact detection is often performed by using a Doppler method. In the Doppler method, the living body is irradiated with a microwave, and the reflected wave from the living body is received; the difference between the oscillation frequency and the reception frequency is converted into an acoustic signal to thereby detect the blood flow. For example, Patent Document 1 describes an ultrasonic diagnostic image processing device using the Doppler method.